The Late Cretaceous was a time of high biodiversity, relatively high global temperatures, and considerable fluctuation in regional sea levels. By quantifying the structure of Late Cretaceous ecological communities, as well as spatial and temporal changes in these systems, we can gain a greater understanding of how ecosystems respond to major environmental shifts, and should provide further insight into predicting extant ecological responses to climate change.

​One method for estimating Cretaceous vertebrate community structure is through the study of vertebrate microfossil bonebeds (VMBs), which are mass accumulations of small teeth, bones, and scales deposited over geologically short timescales in ancient wetland/lake/river environments, and these assemblages are thought to be representative of the average relative abundance of different clades within their contemporaneous ecological communities. A major component of my PhD thesis focused on using data from Late Cretaceous VMBs to test hypotheses of community responses to environmental perturbation.

(how a vertebrate microfossil bonebed appears at the surface, with multiple weathering fossils)

(example of VMB material)

The first of these investigations focused on describing a new VMB assemblage from the lower Belly River Group, during the regressive phase of sea level drop near the boundary between the Foremost and Oldman formations (specifically in the upper Foremost Formation, just below the Taber Coal Zone). The site preserved a transitional fauna containing a mix of marine taxa (like sharks and other chondrichthyans), and taxa with terrestrial affinities or no strong environmental affinity (turtles, dinosaurs, eusuchians, etc). In addition, the site preserved the first record of a large hybodont shark cephalic spine from the Cretaceous of Alberta, and the first record of the ratfish Elasmodus from the Foremost Formation. Comparisons of the site with other sites from the Foremost and Oldman suggested that certain groups (such as lissamphibians and shark) were integral for palaeoenvironmental inference from microsite assemblage data.

This paper was published in the February 2016 issue of the journal Palaeogeography, Palaeoclimatology, Palaeoecology.​

(A hybodont shark cephalic spine, Cullen et al 2016)

(R-vs.Q-mode cluster analysis of VMB data, Cullen et al 2016)

(Taphonomic analysis of VMB materials, Cullen et al 2016)

In the next stage of this project, we combined the data collected above with those from dozens of other VMBs (48 sites total), sampled from two regions approximately 150 km apart and spanning ~5 million years over the stratigraphic extent of the Belly River Group in Alberta, in order to analyze changes in community structure (via relative abundance among different taxa), and how these changes correlated with environmental changes related to transgressive-regressive cycles of regional sea level (as well as other factors). Specifically, we quantified changes in species composition and relative abundance between sites, and performed a series of analyses (including R- vs Q-mode cluster analysis, redundancy analysis, and pairwise relative abundance and similarity analyses) comparing the community trends against abiotic factors such as site sedimentology/deposition, sampling location, stratigraphic position, and palaeoenvironmental setting. A specific hypothesis of interest in these analyses was to test if dinosaurs were particularly sensitive to microhabitat changes driven by shifting altitudinal gradients (i.e. sea level changes), as has been hypothesized by a number of previous studies. Our analyses found that the strongest abiotic factor controlling community structure was palaeoenvironment, with site sedimentology, stratigraphic position within the Belly River Group, and sampling location having lesser impacts. On the broader community scale, the most severe changes in community structure occurred between chondrichthyans and lissamphibians, which showed a strong inverse relationship in relative abundance, particularly during shifts from marine-to-terrestrial or terrestrial-to-marine settings. While that result in of itself is not particularly surprising, it is important in establishing that community responses to environmental perturbations are detectable, and serves as a useful comparison to the dinosaur components of the community, which were largely unaffected in relative abundance by those changes (with the exception of periods at the very bottom and top of the Belly River Group where the system becomes fully marine and consequently almost all terrestrial fossils disappear). The results so far suggest that dinosaurs may not be particularly sensitive to small-scale environmental perturbations or changes in microhabitat.

This paper was published in November 2016 in the journal BMC Ecology, It was an Editor's Pick and BMC Ecology top 10 highlight for 2016, and was the subject of a BMC Series blog.

We are currently expanding these comparisons through the sampling, description, and analysis of additional VMB assemblages. The first step of this process involves sampling sites from the currently unsampled interval from the uppermost Oldman Formation in the Milk River / Manyberries region of southernmost Alberta, as well as collecting more detailed stratigraphic data. Going forward, Going forward we will expand sampling both spatially and temporally, in order to further quantify trends in community structure across environmental gradients, and more specifically to test how these systems respond to more abrupt, larger-scale perturbations such as were seen in the K-Pg mass extinction.

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